PGWFT: A Petri Net Based Grid Workflow Verification and Optimization Toolkit. Graphical workflow modeling tools, such as UML and DAG, can facilitate users to express workflow process logic, but lack of abilities to carry out simulation and correctness checking. In this paper, we propose a service composition oriented grid workflow model and its related six elementary workflow patterns: sequence, condition, iteration, concurrency, synchronization, and triggering. Based on this, we present a Petri net based grid workflow verification and optimization toolkit, called PGWFT, to help common users describe workflow instances and perform verification and optimization analysis. The experimental results show that our workflow verification and optimization mechanisms are feasible and efficient.